Looking to expand payload capabilities of the Proton rocket downwards, its developer proposed smaller, cheaper version of Russia's commercial space workhorse. If ever built, the Proton-Light variant is expected to have the industrial designation 8K82KL.

Upgrades envisioned for Proton-Medium (center) and Proton-Light rockets (right) as of 2016.

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Proton "light" concept

In 2016, Russian space officials announced plans to develop a smaller version of the nation's venerable Proton launch vehicle. The head of Roskosmos Igor Komarov, speaking to the radio station Ekho Moskvy on June 18 said that the Proton-Light variant would be narrowly aimed at particular demands of the commercial market. According to Komarov, an extra variant would make the Proton family more effective and reduce per-kilogram cost of payload delivered into orbit.

Komarov stressed that the Proton would remain in operation for at least seven or nine years, before being replaced with the Angara family.

On June 23, the head of GKNPTs Khrunichev Andrei Kalinovsky announced that the company had been preparing a new addition to the Proton family for a different type of commercial spacecraft, allowing various Proton variants to have their own individual price offers on the market. Kalinovsky did not provide technical details on the new Proton variant, but mentioned that it would not be a drastically new vehicle.

The original concept of the Proton-Light vehicle.

Around the same time, various industry sources revealed the most likely architecture of the Proton-Light rocket. The key change included the removal of the second stage from the standard Proton-M rocket. Instead, the traditional third stage of the Proton-M would be "stretched" to accommodate more propellant and used as the second stage on the Proton-Light. According to some reports, the propulsion system on the second stage could be modified to re-start its engine and boost its payload from an initial parking orbit to a geostationary transfer orbit, GTO. Some estimates showed that under such a scenario, the vehicle could deliver up to five tons of cargo to the GTO after launch from Baikonur.

Another configuration could use a Briz-M space tug as the third stage of the rocket.

It appears that within weeks after the Proton-Light concept first surfaced, the newly proposed architecture of the rocket had to be revised. As it transpired, it would not be enough to compensate for the removal of the second stage by simply stretching the third stage. In addition, the first stage of the rocket, including its core tank and six strap-on tanks, would have to be extended as well, making this "downgrade" more complicated.

In its new configuration, Proton-Light could fill the gap in the payload range left by the Zenit rocket, which itself fell victim to Russia's conflict with Ukraine. The new Proton variant would also be well positioned on the international market, which saw more competition with the arrival of the American Falcon-9 rocket.

However given the extended length of time required for even less radical upgrades of Proton and the official Russian strategy to phase out the vehicle in favor of Angara-5, it is unclear whether it would be possible to justify the Proton-Light development effort. A number of previous proposals to change the shape and size of the Proton-M rocket were deemed too expensive more than a decade earlier in the rocket's operational career.

On July 8, 2016, Kalinovsky told the TASS news agency that he had hoped to have Proton-Light ready for launch as soon as 2018, even though he admitted that the project had been in a "decision stage" and the exact specifications of the rocket had still being worked out. Kalinovsky stressed that the proposal had had a purely commercial nature and was aimed to beat market prices offered by SpaceX.

Proton family to expand further

On Sept. 13, 2016, GKNPTs Khrunichev officially confirmed its plans to expand the Proton family and announced the coming of its additional members.

One new version of the launcher now identified as Proton-Light would be formed by the removal of two out of six strap-on tanks and their respective engines on the first stage and by the addition of an auxiliary fuel tank at the top of its main body. (Due to the smaller amount of fuel on that version, the standard oxidizer tank would have to be left only partially filled.) The fuel from the add-on tank would flow via four pipelines to four strap-on tanks, before reaching its respective engines. Since none of the tanks separate in flight, the new arrangement will require a relatively simple design.

The company promised to introduce that latest configuration of the vehicle in 2019, or one year after the "initial" Proton-Light architecture, which was now renamed Proton-Medium. The new "flavors" of the Proton will be launched from Pad 24 at Site 81 in Baikonur and will still use the standard Briz-M space tug as their third stage.

The lightest version of the Proton rocket could deliver from 3.6 to 4.2 tons of payload to the geostationary transfer orbit, GTO, depending on the additional kick provided by the satellite itself, and insert 1.45 tons of payload directly into the geostationary orbit. The Proton-Medium could deliver from 5.0 to 5.7 tons to GTO and 2.4 tons into the proper geostationary orbit.

Proton-M rocket (left) shown to the same scale with the proposed Proton-Medium (center) and Proton-Light variants.